Frequently, devices which utilize electrodes composed of materials such as carbon and/or graphite are employed for substantial periods of continuous operation. Typically, in many of these devices, such as arc furnaces employed to melt scrap in the steel industry, the electrode is consumed from its tip because of sublimation or for other reasons, such as scrap caving or vibrational stress. As there are constraints as to the length such electrodes may be fabricated, shipped and/or employed, there have been developed electrode segments comprising electrode members and having mating surfaces. These electrode segments are added to the top of the electrode column by the device operator when consumption of the electrode column has occurred to a specified extent. Typically, mechanical means such as threaded pins are employed to join such electrode segments into the electrode column.
However, during operation of the device, the electrode column is frequently subjected to tremendous mechanical vibrations and thermal shocks as well as to strong electromagnetic fields. The stresses created by such shocks, vibrations and electromagnetic field forces may cause the electrode column to disassemble at the electrode segment joints, even where threaded pins are employed. Such disassembly may occur in consequence of either pin and socket breakages or may result from the unscrewing of one electrode segment from a second electrode segment.
To overcome this problem of electrode joint disassembly, it has been proposed in the prior art to cement the pin to the sockets in order to increase the contact surfaces between the pin and the sockets, and to provide a more uniform distribution of stresses in the joint area.
Thus, for example, U.S. Pat. No. 2,510,230 shows an electrode joint construction wherein a low softening point (e.g., 85.degree.-115.degree. C.) binder pitch-cement is disposed in reservoirs drilled in the pin. At the elevated temperatures to which the electrode column is subjected, the pitch-cement melts and flows out of the reservoirs to fill in the gaps between the pin and thread sockets. However, several disadvantages are associated with such pitch-cement reservoirs. For example, because of constraints in the joint area, there is no control over the flow and distribution of the pitch on the pin and electrode socket surfaces. Moreover, forming reservoir holes in the end faces of the electrode segments and/or in the pin requires an expensive drilling step and mechanically weakens the joint. Further, because the pitch-cement employed must flow, it must therefore possess a low viscosity. Consequently, such pitch-cement will possess a relatively low Modified Conradson Carbon value and will generate large amounts of volatiles during carbonization.
U.S. Pat. No. 2,862,748 discloses an electrode column having cemented joints wherein a liquifiable carbonizable composition (preferably a coal tar pitch having a softening point of between 60.degree. and 120.degree. C.) fills the pores of the entire surface of a threaded carbon connecting member to a depth represented by from about 10% to 75% of the total available porosity of said member, the unfilled interior pores being filled with a gas. This member is preferably produced by machining it to the general contour of the electrode socket with which it will engage; immersing the premachined carbon body into a coal-tar pitch having a softening point of 60.degree.-120.degree. C. at a temperature of 250.degree. C. under a pressure of 50 psi until 10-75 percent of the total available pores inwardly from the entire surface have become filled with pitch; cooling the impregnated member; and grinding said impregnated member to its final form. Although the use of such an impregnated pin will increase the torque required to break the joint at room temperature by 3 or 4 times, the multiple grinding steps add increased cost to the process, remove all pitch from the surface of the connecting member, and also remove most of the impregnated pitch, thereby only permitting a slight increase in strength. Moreover, such low softening point pitch will produce a relatively large amount of volatiles upon carbonization, thus leaving less carbon to cement the bond between the electrode segments.
U.S. Pat. No. 4,395,299 discloses the use of a mesophase pitch having a softening point above 240.degree. C. and a melting point above 300.degree. C. as an electrode cement. The solid pitch particles are placed at the electrode joint surface and the joint is formed by heating at 700.degree. C. to 3000.degree. C. under 20 to 1000 psig pressure which is normal to the bonding surfaces under a vacuum or in an inert atmosphere. However, problems exist in applying such pitch-cement to the electrode joint as gravity will cause the pitch particles to fall, said particles being neither adhered to nor impregnated within the joint surface. In addition increased costs are encountered in providing the necessary vacuum or inert atmosphere for joint formation.
It would therefore be desirable to have a component for incorporation into an electrode joint, as well as a process for the formation of an electrode joint, which joint exhibits increased strength, and which process is safe and economic.
Accordingly, it is an object of this invention to provide an electrode member for incorporation into an electrode joint, which joint will demonstrate increased strength.
It is another object of this invention to provide a process for producing such an electrode member.
It is yet another object of this invention to provide a process for the production of an electrode joint, which process is economic, safe and which will produce a joint having increased strength.
The foregoing and additional objects will become apparent from the following description and examples and the attached drawing.